The present invention relates generally to methods and systems for electrodeposition of silver, and more particularly to an improved, cyanide-free method for silver deposition or recovery.
In a process for electroplating a metal, a part to be plated is usually connected to the negative terminal of a direct current (DC) power source. The positive terminal is connected either to an insoluble anode or to an anode constructed of the metal part to be plated. The part to be plated and the anode are immersed in an electroplating solution containing ions of the plating metal. Plating metal is dissolved at the anode (when soluble anodes are used) and deposited (plated) on the part to be plated (at the cathode). Conventional electroplating solutions for silver are generally of the aqueous type comprising cyanide in basic solution, and may contain additives (addition agents) to enhance the deposition process and/or the appearance or performance characteristics of the coating.
The invention provides an improved, cyanide-free method for electrodepositing silver metal onto current conducting substrates in either a silver plating process or a silver recovery process from silver bearing scrap. A substrate to be plated is connected to the negative terminal of a DC power supply, and the positive terminal is connected to the solution containment vessel, to an insoluble anode, or to a silver anode in electrical contact with an aqueous electrolyte solution of ethylenediamine and an alkali hydroxide of selected basic solution. Addition agents, corrosion inhibitors and other electrolytes common to conventional cyanide silver plating solutions may be added. In practicing the invention, silver is dissolved at the anode without damaging steels similarly exposed to conventional (basic cyanide) electroplating solutions. The invention may be particularly useful in continuous silver plating processes. Silver deposition may be achieved in the presence of contaminants in solution typically found and/or are acceptable in conventional silver plating solutions, e.g. sodium, iron, nickel and copper. Certain contaminant metals, to include but not limited to, copper, gold, nickel, iron, tin and zinc may be dissolved at the anode. Tolerances to these metals and others (e.g., sodium, potassium, calcium, etc.) are similar to those observed in cyanide Plating baths. Significantly, nickel does not plate out substantially in the practice of the invention. Applied voltages required to achieve deposition are similar to those required in cyanide baths, e.g., higher silver loadings and higher electrolyte concentrations require less applied voltage to maintain acceptable current densities. When used in a silver recovery process, the purity of the recovered silver may be enhanced by controlling solution purity and using low operating current densities. Spent electroplating solutions may be treated for extraneous metals and disposed of as biodegradable.
It is therefore a principal object of the invention to provide an improved method for electroplating silver.
It is a further object of the invention to provide an improved cyanide-free plating solution for electroplating silver.
It is yet another object of the invention to provide method and electroplating solution for the recovery of silver from silver bearing scrap.
These and other objects of the invention will become apparent as a detailed description of representative embodiments proceeds.